Lapse Rates and Stability

Adiabatic Processes

• no heat is added or removed from air
• rising air >> lower pressure >> expands >> temperature decreases
• sinking air >> higher pressure >> compresses >> temperature increases

Lapse Rates

• Dry Adiabatic Lapse Rate (DALR): 3 C / 1000 ft
• Saturated Adiabatic Lapse Rate (SALR): 1.5 C / 1000 ft
• Average: 2 C / 1000 ft

• Environmental Lapse Rate (ELR): indicates the temperature of the surrounding air; comparison to lapse rate of air parcel determines if it is stable or unstable
• shallow lapse rate compared to SALR: absolute stability
• lapse rate between DALR and SALR: conditional instability
• steeper lapse rate than DALR: absolute instability

Characteristics of Stable Air

• sustained low visibility (i.e. haze layers, drizzle, fog)
• continuous precipitation
• strato-form clouds (layers)
• steady winds

Characteristics of Unstable Air

• good visibility
• showery precipitation
• cumulo-form clouds (vertical development)
• gusty winds

DO YOU KNOW... why rising saturated air cool less rapidly than rising unsaturated air?

• heat is released during the condensation of water vapour

How High Are We, REALLY?

True Altitude was something that always seemed to trip us up in ground school, but it's not actually that difficult.  In fact, if you read the instructions on your E6B, you're already more than half-way there!

True Altitude is particularly important when there are abnormally cold temperatures.  Usually the scenarios presented involve leveling off at a MEA, flying over a ridge, and realizing that you don't have nearly the amount of altitude clearance as you might have expected because it's -40. 

4 simple steps:

1. set air temperature over pressure altitude (make sure you're using the correct side!)
2. read true altitude over calibrated altitude scale (calibrated = planned - height of altimeter source)
3. add result of (2.) to height of altimeter source
4. subtract height of ridge to get ridge clearance

I'm tagging this both SAMRA and SARON because it fits in both Meteorology and Flight Operations.  So it could come up anywhere!